The present invention relates generally to the quantitative evaluation of total fissile and total fertile nuclide content in samples, and more particularly to simultaneous photon and neutron interrogation of a sample coupled with the measurement of resulting prompt and delayed neutron emission as the basis for analysis of the totality of the fissile material present in the form of .sup.233 U, .sup.235 U or .sup.239 Pu, and the totality of fertile material present in the form of .sup.232 Th and .sup.238 U in the sample under investigation.
Current U.S. Department of Energy guidelines for the management of transuranic waste have created a need for instrumentation to monitor such wastes at the 10 nCi/g level of fissile nuclides in the presence of fertile material. Solely passive systems which rely on the detection of gamma rays or neutrons from the decay or spontaneous fission of transuranic wastes are generally not suitable because of (1) the attenuation of high density matrix material with the attendant loss of sensitivity, and (2) interference from gamma and alpha emitting contaminants in the matrices. Active interrogation methods, wherein the waste is probed by an externally generated neutron or photon pulse, which cause fission in many of the nuclides present, or a combination of active and passive methods, overcome many of the problems associated with the passive methods alone. Two currently used active methods are: (1) photofission where a high energy photon beam induces fission in the waste sample and fission and delayed neutrons thereby produced are detected, and (2) thermal neutron fission where fast neutrons from a pulse source, after moderation, induce fissions in those nuclides present in the sample which are fissile. Active assay systems based on these methods have been used but neither approach itself is entirely adequate. Photofission offers good sensitivity for a large number of transuranic waste samples, but because of similarity of photofission cross-sections, identification of specific nuclides or classes of nuclides is difficult. For example, the important fissile and fertile groups cannot readily be distinguished. Thermal neutrons, on the other hand, offer very high sensitivity for fissile elements but essentially none for fertile elements. A combination of the two methods would be most desirable but technical complexities such as the need for two pulsed sources, together with longer assay times, have made such an analytical system impracticable.
A combination of neutron and photon interrogation offers several distinct advantages over either applied alone, including a direct and unequivocable separation of fissile and fertile nuclides within the sample under investigation. The method and apparatus of the instant invention demonstrates that dual interrogation can be achieved using an electron linear accelerator (LINAC) as a pulsed source for both photons and neutrons. Moreover, both interrogations are initiated during each pulse of the LINAC, and the resulting prompt and delayed neutrons can be monitored with the same detection system.
It is known in the art that high energy gamma radiation is produced as a result of bremsstrahlung in a heavy-metal target placed in the path of a high energy electron beam. The production of neutrons through the use of electron beams is also known, and occurs when the high energy gamma photons subsequently pass through additional layers of a target causing neutrons to be emitted in (.gamma.,n) processes. "Efficient Neutron Production Using Low-Energy Electron Beams," by C. D. Bowman, Nucl. Sci. Eng., 75, 12 (1980). However, the combination of neutrons and gamma radiation produced from a single source and used for analysis of transuranic waste samples for total fissile and total fertile nuclides present has not been reported.
Most of the photons will pass into the volume of the waste sample where some will cause photofission. Prompt neutrons emitted from the photofission of either fissile or fertile nuclides will not be distinguishable from photoneutrons that are formed in the materials of the chamber containing the sample under investigation and/or the matrix materials which contain these fissile and fertile nuclides. However, delayed neutrons from photofission will be emitted on a continual basis during the whole period between LINAC pulses. Photoneutrons and prompt photofission neutrons will thermalize in a few tens of microseconds and will persist as thermals for hundreds of microseconds, during which time they will generate thermal neutron fissions among the fissile transuranic nuclides that may be present. Therefore, fission neutrons from thermal fission are separated in time from the photoneutrons, and can serve, along with delayed neutrons, as a quantitative signature. Essentially then, after an initial burst of photoneutrons and neutrons from photofission, the bulk of the fast prompt neutrons derive from thermal fission of fissile materials. Subsequent to the emission of these prompt neutrons are the emission of delayed neutrons which derive from both photofission of fertile and fissile material as well as delayed neutrons from thermal neutron fission of fissile nuclides. If the latter contribution to the total delayed neutron flux is made small, the delayed neutron emission is representative of the photofission events only. To achieve this result, some iteration of the interrogation neutron flux may be necessary when analyzing samples of completely unknown fissile content. This flux can be varied by choosing different target materials for the (.gamma.,n) source.
Thus, in the instant invention, the events detected following a single LINAC pulse are separable into neutron fission (prompt fission neutrons) and photofission (delayed neutrons) events. This data can then be analyzed to yield the individual quantities of fertile and fissile isotopes present.